1. Technical Field
The present invention relates to a resonator element, a resonator, an electronic device, an electronic apparatus, and a moving object.
2. Related Art
Since an AT cut quartz crystal resonator that vibrates in thickness shear vibration as a vibration mode of main vibration is suitable for miniaturization and increasing a frequency and has an excellent frequency-temperature characteristic, it has been used in an oscillator and an electronic apparatus. Particularly, in recent years, as the processing speed of transmission communication equipment or OA equipment is increased or the volume of communication data and the processing capacity are increased, there has been a strong request for increasing a frequency for the AT cut quartz crystal resonator which is a reference frequency signal source used therein.
Generally, since a resonant frequency and the thickness of a quartz crystal plate have an inverse relationship in the AT cut quartz crystal resonator using the thickness shear vibration as main vibration, it is possible to increase the frequency by reducing the thickness of the quartz crystal plate.
JP-A-11-284484 discloses an AT cut quartz crystal resonator of a so-called reverse mesa structure in which a concave portion is formed in a part of a main surface of a quartz crystal plate so as to reduce the film thickness of a vibration region and increase the frequency. It is described that a good frequency-temperature characteristic is obtained by setting the ratio between the plate thickness of the vibration region of the quartz crystal plate and the film thickness obtained by converting the electrode formed on the front and back of the vibration region into a quartz crystal density to a range of 7% to 13%, in the AT cut quartz crystal resonator in which a fundamental wave vibration vibrates at a resonant frequency of 300 MHz or more.
In addition, similarly to JP-A-11-284484, JP-A-2005-203858 discloses an AT cut quartz crystal resonator of a reverse mesa structure in which a fundamental wave vibration vibrates at a resonant frequency of 300 MHz or more. In the AT cut quartz crystal resonator, it is possible to prevent a frequency change before and after reflow, and a change in a frequency-temperature characteristic, by setting the ratio between the plate thickness of the vibration region of a quartz crystal plate and the electrode thickness on one side of the electrodes formed on the front and back of the vibration region to be 0.014 or less or 0.012 or less (the ratio between the plate thickness of the vibration region of the quartz crystal plate and the film thickness obtained by converting the electrode film thickness into a quartz crystal density is 19.2% or less or 16.5% or less).
However, note that if the resonant frequency of the fundamental wave vibration is increased, especially, if the resonant frequency is set to 200 MHz or more, there is a problem that it is necessary to reduce a CI (quartz crystal impedance=equivalent resistance of the quartz crystal resonator) value in order to drive the quartz crystal resonator at the resonant frequency of the fundamental wave vibration, in the oscillation circuit equipped with the quartz crystal resonator having the above-described structure. Especially, if the resonant frequency is set to a high frequency of 200 MHz or more, the film thickness of the excitation electrode and the lead electrode which are formed on the quartz crystal plate significantly affects the CI value of the main vibration. If a mode of trapping only the main vibration of the quartz crystal resonator is set, it is necessary to reduce the thickness of the electrode film; whereas if the thickness of the electrode film is set to 100 nm or less, a sheet resistance is rapidly increased, such that a large ohmic loss (resistance loss due to surface resistance) occurs at the excitation electrode portion and the lead electrode portion, this results in a problem of the CI value of the quartz crystal resonator being increased.
However, if the thickness of the electrode is increased in order to reduce the ohmic loss of the electrode film, spurious vibration is generated by the vibrations of many in-harmonic modes being trapped in addition to the main vibration, the CI value of spurious vibration close to the main vibration becomes smaller than the CI value of the main vibration depending on conditions, and thus there is a problem of the oscillation circuit oscillating at the resonant frequency of the spurious vibration.